There have been a variety of prior art mattresses and other support surfaces which have employed fluids such as water or air to provide a resilient surface. One of the problems with such support surfaces is dampening the flow of the fluid as the load residing on the support surface shifts. A second problem results from the tendency of such support structures to drastically deform, causing the load to bottom out or vacillate in response to a heavy load which is rapidly applied.
The former problem associated with dampening has been in part overcome by the use of fluid-dampening inserts such as taught in U.S. Pat. No. 4,280,235. The latter problem of bottoming out or vacillating in response to a rapidly applied heavy load has been in part overcome by the use of paired fluid chambers such as taught in U.S. Pat. No. 3,722,875.
The problem of local deformation in mattresses can be a particular problem in beds, since the deformation of the edge of the bed can result in a bed where it is difficult to get off the mattress and out of bed. This problem has in part been overcome by the teaching of U.S. Pat. No. 4,827,546, which teaches employing a series of interconnected fluid containing bellows in combination with a network of springs to provide a composite support structure.
Another approach to providing a variable character mattress is taught in U.S. Pat. Nos. 5,060,328; 5,159,725; and 5,335,380. These patents teach the use of bellows to provide a spring insert in water beds to change the support characteristics in selected regions. The teaching of these patents is limited to the use of water as the fluid, and the bellows employ restriction of the fluid outflow from the bellows to provide resistance to loads. In one embodiment, stacked bellows are employed, and adjusting the restriction of flow between the stacked bellows provides limited ability to adjust the resistance. Adjustment of the device by the user appears to be difficult, and it is unclear from these patents whether such adjustment is in fact practical.
None of these devices provide a support surface which is capable of providing a resilient response to loads where the stiffness of the response has a dual character as a function of displacement, having an initial soft response to displacement and a subsequent stiffer response to provide proper biomechanical support. Additionally, these surfaces are either not adjustable or adjustable to only a limited degree with respect to the characteristic of the resistant force that can be generated.
Thus there is a need for a support surface which can provide a two-staged resilient response of the surface to an applied load, and there is a need for a support surface where the character of the response can be readily adjusted.